1. Field of the Invention
The present invention relates to an optical scanner which performs scans by the scanning of an optical beam, and, in particular, to an optical scanning device having a structure in which a micro mirror which is supported by torsion bars is made to oscillate so as to cause the direction of an optical beam to change.
2. Background Art
In recent years, optical scanners which scan optical beams of laser light or the like have been used as optical instruments such as bar code readers, laser printers, and head mounted displays, or as the optical intake devices of input devices such as infrared cameras and the like. Optical scanners having a structure in which a micro mirror obtained via silicon micromachining technology is oscillated have been proposed for this type of optical scanner. For example, the structure described in Japanese Unexamined Patent Application, First Publication No. H07-65098 (Patent document 1) is known (referred to below as ‘Conventional technology 1’). As shown in FIG. 19, this optical scanner irradiates light which is emitted from a light source 100 and reflected by a mirror portion 101 onto a detection object 102, and then vibrates the mirror portion 101 so that the light is scanned in a predetermined direction of the detection object 102, and is provided with two mutually parallel drive sources 103 which are formed as cantilevered beams with one end respectively thereof formed as a fixed end and which perform bending operations, a linking component 104 which links together the free end sides of the two drive sources 103, a torsional deformation component 105 which extends from a center portion of the linking component 104, and the mirror portion 101 which is provided on this torsional deformation component 105. The center of gravity of the mirror portion 101 is made to sit on the torsion center axis of the torsional deformation component 105. If the two drive sources 103 are driven, for example, by a bimorph structure on which a piezoelectric material has been adhered, and are vibrated in antiphase, then torsional vibration is induced in the torsional deformation component 105, and the two drive sources are driven at the resonance frequency of the torsional deformation component 105. As a result, it is possible to vibrate the mirror portion 101 over a sizable amplitude.
Moreover, as shown in FIG. 20, the optical scanner described in Japanese Unexamined Patent Application, First Publication No. H04-95917 (Patent document 2, referred to below as ‘Conventional technology 2’) is a scanner in which a mirror surface 111 is formed by a surface of a vibrator 110 having two elastic deformation modes, namely, a bending deformation mode and a torsional deformation mode, and in which this vibrator 110 is vibrated at the respective resonance frequencies of the two modes. Optical beams irradiated towards the mirror surface 111 of the vibrator 110 are reflected by that mirror surface 111 so that the light is scanned in two directions. If the vibrator 110 is vibrated in a single mode, then this scanner becomes a one-dimensional scanning optical scanner.
Moreover, as an optical scanner in which a micro mirror obtained by means of silicon micromachining technology is oscillated, the structure described in Japanese Unexamined Patent Application, First Publication No. H10-197819 (Patent document 3) is known (referred to below as ‘Conventional technology 3’).
As shown in FIG. 21, this optical scanner is provided with a plate-shaped micro mirror 121 which is used to reflect light, a pair of rotation supporting bodies 122 which are positioned on a straight line and support both sides of the micro mirror 121, a frame portion 123 to which the pair of rotation supporting bodies 122 are connected and which surrounds the periphery of the mirror 121, and a piezoelectric element 124 which applies translational motion to the frame portion 123. In addition, this optical scanner is structured such that the center of gravity of the mirror 121 is located at a position away from the straight line connecting together the pair of rotation supporting bodies 122.
When voltage is applied to the piezoelectric element 124, the piezoelectric element 124 is made to expand and contract, so as to vibrate in the Z axial direction. This vibration is transmitted to the frame portion 123. When the micro mirror 121 is made to move relative to the driven frame portion 123 and the vibration component in the Z axial direction is transmitted to the micro mirror 121, because the micro mirror 121 has a left-right asymmetrical mass component relative to the axis formed by the X axis rotation supporting bodies 122, rotational moment is generated in the micro mirror 121 centered on the X axis rotation supporting bodies 122. In this manner, the translational motion which has been applied to the frame portion 123 by the piezoelectric element 124 is transformed into rotational motion centering on the X axis rotation supporting bodies 122 of the micro mirror 121.
Moreover, as shown in FIG. 22, an optical scanning device is also described in Japanese Unexamined Patent Application, First Publication No. H10-104543 (Patent document 4, referred to below as ‘Conventional technology 4’). In this optical scanning device, beam portions 133 and 133 extend in mutually opposite directions from both sides of a movable portion 132 in a vibrator 131, and are connected to two arm portions 134 and 134 of a fixed portion 136. Piezoelectric thin films 135 and 135 are provided respectively on the arm portions 134 and 134 of the fixed portion 136, and these piezoelectric thin films 135 and 135 are driven by the same signal which includes higher order vibration frequencies.
In the optical scanning devices of the above-described prior technologies, in order to achieve a small-size, portable laser projector or the like, it is necessary to position the above-described optical scanning device in a compact arrangement together with a laser light source and other optical systems, so that it is essential that such apparatuses are designed to be as small as possible. In order to achieve this, it is possible to miniaturize the optical scanning device using Si micro machining and the like, however, in contrast, in the case of a laser projector system which performs a single mirror scan, because the optical aperture width is determined by the size of the mirror, if this optical aperture width is too small, then it is not possible to achieve a sufficiently small spot size on the projection surface. As a result, there is a considerable deterioration in image resolution. Because of this, it is necessary for the size of the mirror to be greater than or equal to at least 1 mmφ, and depending on the application, a surface whose area is greater than or equal to 5 mm square is considered necessary. In this case, because the length of the hinge which supports the mirror portion is added to the mirror size, the size of those structural portions of the optical scanning device which generate torsional resonance ends up being greater than or equal to at least 5 mm square, and in some cases, greater than or equal to 1 cm which hinders attempts to make the size of the device any smaller. This is a serious problem as it makes it difficult for the scan angle of the optical scanning device to be made greater than or equal to 30°, and in the case of a low-scanning speed optical scanning device having a resonance frequency that is less than or equal to 100 Hz which is used for the vertical scanning of a two-dimensional scan, makes it difficult for the design to be made more compact (referred to below as Problem 1).
Moreover, in the design of an optical scanning device having a resonance frequency that is greater than or equal to 10 kHz in which the length of the torsion hinge is comparatively short, when this is made to resonate by a large driving force and the mirror portion is scanned using a large torsion angle that is greater than or equal to 20°, because the torsion angle of the torsion bars per unit length increases considerably, if the torsion bars are made from a metal material or the like, the problem has occurred that because of metal fatigue there is an abrupt deterioration in performance. Moreover, when the torsion bars are made from a brittle material such as silicon monocrystals, in order to achieve a large scan angle, there is a limit in the torsion angle per unit length. Because of this, it is necessary to design the hinge length to be comparatively long, and reducing the size of the mirror resonance structural portions and of the overall optical scanning device continues to remain a difficult design problem (referred to below as Problem 2).
If, for example, the spring constant of the elastic deformation portion (i.e., the torsion bar portions) in Conventional technology 1 is taken as k, and the moment around the rotation axis (i.e., the Y axis or the Z axis) is taken as I, then the resonance frequency f in a vibrator 1 can be expressed by the following formula.
                    f        =                              1                          2              ⁢                                                          ⁢              π                                ⁢                                    k              I                                                          (        1        )            
The spring constant in the bending deformation mode (in a θB direction) of the elastic deformation portion is taken as kB, while the spring constant in the torsion deformation mode (in a θT direction) is taken as kT. If the spring constant k in Formula (1) is replaced by these spring constants kB and kT, then Formula (1) shows the resonance frequency fB in the bending deformation mode, and shows the resonance frequency fT in the torsion deformation mode, and the spring constant kB in the bending deformation mode is expressed by the following formula.
                              K          B                =                              Ewt            3                                4            ⁢                                                  ⁢            L                                              (        2        )            
Here, E is Young's modulus, w is the width (i.e., the Length in the Y (direction) or the elastic deformation potion, t is the thickness (i.e., the length in the X direction) of the elastic deformation portion, and L is the length (i.e., the length in the Z direction) of the elastic deformation potion.
The spring constant kT in the torsion deformation mode is expressed by the following formula.
                              K          T                =                                            G              ⁢                                                          ⁢              β              ⁢                                                          ⁢                              wt                3                                                    12              ⁢                                                          ⁢              L                                ⁢                      (                          t              <              w                        )                                              (        3        )            
Here, G is the modulus of transverse elasticity, and β is a coefficient relating to the shape of the cross section. In Formula (3), more typically, w represents the length of a long side of the cross section of the elastic deformation portion, and t represents the length of a short side of the same cross section.
It is understood from Formula (1) that, as a result of the spring constant k changing, the resonance frequency of the vibrator is changed.
Moreover, in actual devices in which the above-described optical scanning devices are used such as laser projectors and barcode readers, because of the necessity for two-dimensional scans or in order to obtain a reduction in size, it is necessary not only to reduce the size of the optical scanning device itself, but to additionally create designs in which various combinations of reflection mirrors and the like are used in order to modify the optical path. However, each time light is reflected onto one of the respective minor portions, the overall light amount is decreased by the absorptivity of that mirror portion, and there is a deterioration in the projected image and in the luminance of the optical beams. In particular, when the optical scanning device is used in a portable device, increasing the amount of light from the light source, and consequently, securing sufficient voltage source capacity have proven to be sizable problems (referred to below as Problem 3).
Moreover, the optical scanning device of the above-described Conventional technology 4 has the drawback that a large torsion angle cannot be formed in the movable portion 132.
Namely, if a piezoelectric film is formed in the two narrow-width cantilever beam portions which support the two torsion bars protruding from the frame portion, then the rigidity of this portion increases and vibration which is induced in the piezoelectric film is not transmitted efficiently to the torsion bars. As a result, the torsional vibration of the mirror becomes smaller. Moreover, unless the vibration characteristics of the vibration source portion formed by the two cantilever beam portions and the piezoelectric film which is formed thereon are matched precisely, then the vibration amplitude of the torsional vibration of the mirror becomes suppressed and, at the same time as this, a vibration mode other than torsional vibration is superimposed thereon so that accurate laser beam scanning cannot be achieved. Furthermore, in order to increase the drive force for the mirror by increasing the surface area of the piezoelectric film portion, it is necessary to increase the width of the cantilever beam portions. Because of this, an unnecessary two-dimensional vibration mode is generated in the same cantilever beam portion, so that at the same time as the vibration amplitude of the torsional vibration of the mirror is restricted, a vibration mode other than the torsional vibration is superimposed thereon. As a result, the problem arises that it is not possible to achieve accurate laser beam scanning. Moreover, because the width of the cantilever beams is restricted to a narrow width, the formation of the top portion electrodes which are used to drive the piezoelectric film formed on this portion is made more difficult because of the narrow width, so that problems arise such as the yield during production being greatly affected (referred to below as Problem 4).
FIG. 23 shows the same case as that of Conventional technology 4, and shows a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from a frame portion. The drive efficiency of the mirror portion scan angle was checked by a simulation calculation. The surface where Y=0was taken as a plane of symmetry, and half of this was used as a model.
FIG. 24 shows the torsion angle of a mirror having a structure in which a piezoelectric film is formed on two narrow-width cantilever beam portions which support two torsion bars which protrude from the frame portion shown in FIG. 23. The drive voltage was set at 1 V, while the characteristics of a PZT-5A which are typical parameters were used for the electrical characteristics of the piezoelectric body, while SUS 304 characteristics were used for the material of the scanner frame main body. The torsion angle of the mirror portion was small at only 0.63°.